Process for upgrading vacuum tubes



Oct. 4, 1966 H. v. NEHER 3,276,836

PROcEss FOR UPGRADING VACUUM TUBES Filed Dec. 9, 1965 05; E QRQM +3 44.

United States Patent 3,276,836 PROCESS FOR UPGRADING VACUUM TUBES HenryVictor Neher, Pasadena, Calif., assignor to Victoreen InstrumentCompany, Cleveland, Ohio, a company of Ohio Filed Dec. 9, 1963. Ser. No.329,153 3 Claims. (Cl. 316-2) This invention has to do with measuringand improving the quality of vacuum tubes, with particular reference tothe amount of residual gas that is present in such tubes.

More particularly, the invention provides procedures for measuring thepressure of residual gas within a vacuum tube and for pumping the tubeto virtually eliminate such residual gas, all without opening the tubeand without requiring any special equipment within the tube.

In the commercial manufacture of vacuum tubes, it is not feasible toproduce as high a vacuum within the tube envelope as is desirable forsome tube applications. In normal manufacture, vacuum tubes are pumpeddown to a pressure of about 10* mm. Hg and then sealed off. Even if thepumping technique could be improved on a production basis, the act ofheating the glass tubulation to seal off the tube releases a smallamount of gas into the tube, sharply limiting the resulting vacuum. Someof the residual gas can be cleaned up by providing in each tube aquantity of getter, such as BaAl, that is I electrically heated andvaporized after the tube has been sealed ofi. However, the vacuumactually produced by such known and customary techniques, typically l0to 10- mm. Hg, is not fully satisfactory for all purposes. Moreover,during normal use of the tube gases tend to escape from the metal, glassand other materials inside the tube, further lowering the vacuum.

Treatment of finished tubes to improve their vacuum is particularlydesirable in the special vacuum tubes of electrometer type which areused primarily to measure extremely small currents, say of the order of1() amperes. Any residual gas inside the tube tends to become ionized bythe electric fields between tube electrodes, and the flow of such ionsproduces spurious currents that afiect such measurements. Even a verylow pressure of residual gas in an electrometer tube can cause avariable grid current of the same order of magnitude as the currentbeing measured. Although such spurious grid current can be partiallycontrolled by limiting all tube voltages to values less than theionization potential of the residual gas, the performance of the tube isthereby severely limited. Moreover, the residual gas tends to causeprogressive deterioration of the cathode surface, so that an instrumentin which the tube is used requires frequent recalibration.

A primary object of the present invention is to provide a process fortreating complete vacuum tubes to effectively eliminate the residual gasthat they normally contain. That is accomplished without opening thetube envelope, and without requiring any special chemical or physicalapparatus within the tube.

In a known procedure for obtaining a very high vacuum, known as ionpumping, special electrodes are provided in the chamber to be evacuated.After preliminary evacuation, an intense electric field is producedbetween those electrodes, and a strong magnetic field is superimposed,parallel to the' electric field. The action of those fields, incombination with the specially provided electrode surfaces, produces aslow but progressive disappearance of the gas from the chamber.Apparently stray electrons present in the chamber are accelerated by theimposed fields, ionizing the gas by collision; and the gas ions are thendriven into the electrode surfaces with sufiicient energy to becomebound.

In accordance with the present invention, it has been discovered thation pumping can be carried out effectively in tetrode or pentode vacuumtubes of conventional design and construction without provision of anyspecial electrodes. An electric field of suitable form and intensity forthat purpose can be produced by direct connection of selected tubeelectrodes to the opposite terminals of a suitable source of directcurrent voltage. In accordance with the invention, the tube is thenplaced in a magnetic field in such orientation that the lines ofmagnetic force are essentially perpendicular to the tube plate.

In preferred form of the invention the entire vacuum tube is heatedduring the ion pumping procedure to an elevated temperature, typicallyof the order to to 300 C.

A further aspect of the present invention provides a process formeasuring the actual pressure of residual gas existing within a tetrodeor pentode vacuum tube of conventional construction. That isaccomplished, like the ion pumping procedure, without opening the tubeenvelope or otherwise affecting the tube, and without requiringprovision of any special structure within the tube.

For that purpose, the tube filament is heated, typically by applicationof the normal filament voltage; a positive voltage, such as 100 volts,is applied to the screen grid to draw electrons from the filament, and anegative bias is applied to the control grid of sufiicient value tolimit the screen grid current to a suitable value. The tube plate ismaintained at a moderate negative potential with respect to thefilament. A sensitive current measuring device is connected in serieswith the tube plate. With that arrangement, the observed current flowingfrom the plate is due to positive ions reaching the plate, and isessentially a linear measure of the residual gas pressure within thetube. That method of measuring the gas pressure is particularly usefulfor monitoring the progressive improvement in the tube vacuum producedby the ion pumping procedure described above.

A full understanding of the invention, and of its further objects andadvantages, will be had from the following description of certaindetailed illustrative procedures by which it may be carried out. Theparticulars of that description and of the accompanying drawings whichform a part of it, are intended only as illustration, and not as alimitation upon the scope of the invention, which is defined in theappended claims.

In the drawings:

' FIG. 1 is a schematic diagram representing illustrative circuitarrangements for treating a completed vacuum tube to improve its vacuumin accordance with the present invention;

FIG. 2 is a schematic elevation representing a vacuum tube undergoingsuch treatment; and

FIG. 3 is a schematic diagram representing illustrative circuitarrangements for measuring the residual gas pressure in a completedvacuum tube in accordance with the present lnvention.

In each of FIGS. 1 and 3 a conventional vacuum tube is representedschematically at 10, with cathode 12, control grid 14, screen grid 16,suppressor grid 18 and plate 20. The cathode is shown as a filament, butmay, alternatively, be of heater type. The suppressor grid is shown'illustratively connected internally of the tube envelope extend betweenthe pole faces of the magnet, as indicated at 30, the sense of the fieldbeing unimportant for the present purpose. Tube is supported in themagnetic field 30 in such orientation that the lines of magnetic forcepass through the tube transversely of the tube plate and preferablysubstantially perpendicularly. The intensity of magnetic field 30 ispreferably as high as can be applied economically without danger ofdeflecting the tube elements permanenntly or causing contact of adjacentelements. A field of the order of 2500 gauss is ordinarily satisfactory.

A source of direct current voltage is indicated schematically at 34,typically comprising a conventional transformer and rectifier fortransforming alternating current power from a source 35 into directcurrent power of the desired voltage at the negative and positive outputterminals 36 and 37, respectively. The output voltage from source 34 ispreferably of the order of 1000 volts. Voltages much below about 500volts are found to be relatively ineffective for the present purpose,while voltages much above about 2000 volts involve danger of arcingwithin the vacuum tube.

In the illustrative circuit of FIG. 1, negative terminal 36 of directcurrent voltage source 34 is connected via the line 38 to plate 20 andalso to control grid 14 of the vacuum tube; and positive terminal 37 ofthe voltage source is connected via the line 39 to screen grid 16 of thetube. Protective resistances 41 and 42, typically of the order of lmegohm, are preferably inserted in lines 38 and 39. Suppressor grid 18and filament 12 may be allowed to float, as shown, or one or both ofthem may be connected to a positive potential, such, for example, as theline 39.

In carrying out the present invention, the tube is treated bymaintaining the described connections, or their equivalent, and amagnetic field such as 30 for an extended time period of the order of 10to 50 hours. During that treatment any electrons appearing between thecontrol grid and plate of the tube, from whatever source, are stronglyattracted toward the positive screen grid. Most such electrons passthrough the screen grid without striking it, and are again attractedtoward it, leading to an oscillatory motion back and forth through theapertures of the screen grid. The presence of magnetic field 30 causessuch electrons to follow a path that is a helix or spiral about thelines of magnetic force and that is therefore far longer than therelatively straight path that would otherwise be followed. Any gasmolecules that are present in the tube tend to be bombarded by such fastmoving electrons and ionized. The resulting positive ions are attractedto the strongly negative plate and control grid, which they strike withsuflicient velocity to have a good probability of becoming bound. Thenumber of free gas molecules is thus progressively reduced .by the.described tube treatment.

It has been discovered, further, that the eflectiveness of the describedtreatment can be considerably increased by maintaining the vacuum tubeduring the treatment at an elevated temperature, such as from about 100to about 300 C. That can be accomplished by surrounding the tubeenvelope by a furnace enclosure, indicated at 50 in FIG. 2. Thatenclosure may be made of asbestos in which is embedded an electricalwinding 52 of high resistance material, such as Nichrome, for example.The winding is connected to electric power source 35 via a controlapparatus, indicated schematically at 54, which is adjustable to varythe current in winding 52 and thereby maintain the vacuum tube at thedesired temperature. Adjustment of control 54 is facilitated byproviding in furnace 50 a temperature indicating device, such as thethermometer 58. A particular advantage of maintaining an elevatedtemperature during the described tube treatment is that the resultingimprovement in the tube vacuum tends to be more stable during'subsequentuse of the tube than when the treatment'is carried out at normaltemperature. i

In FIG. 3, tube 10 is connected in an illustrative circuit for measuringthe residual gas pressure in accordance with the present invention. Thefilament 12 or the cathode heater is supplied with essentially thenormal heating voltage from a source indicated schematically as thebattery 60. The relative potentials to be described for supply to thevarious tube electrodes may be obtained in any suitable manner. Themultiple-brush potentiometer structure indicated at 62, with directcurrent voltage source 64 connected ,to its terminals, is illustrative.The tube cathode is connected to an intermediate point of potentiometer62,?which point will be considered as zero potential, and may begrounded as. indicated at 65.

Plate 20 is connected to a point of. potentiometer 62 that is negativewith respect to the cathode by a potential of the order of 10 volts, aspecific illustrative value being 9 volts. Screen grid 16 is connectedto a point of potentiometer ,62 that is strongly positive with respectto the cathode by a potential of the order of 100 volts, a typicalspecific value being volts. The screen grid then functions as aconventional plate, attracting electrons released from the cathode.Control grid 14 is connected to a point of potentiometer 62 that issufficiently negative with respect to the cathode to limit he currenflawing to the screen grid to a value suitable to the tube in question.For a miniature tube of electrometer type, for example, a suitable valueis to 200 microamperes, which require a negative bias of the order of-50 volts on the control grid, a specific illustrative value being -60volts.

With the described connections, electrons are eflectively prevented fromreaching tube plate 20 by its negative potential, so that the only platecurrent is due to positive ions produced within the tube, primarily byelectrons accelerated toward the positive screen grid. The density ofsuch ions,'for a given valuev of the electron flow to the screen grid,is essentially proportional to the density of gas molecules present tobe ionized. Hence the magnitude of the current flowing from the plateprovides an accurate measure of the residual gas pressure within thetube. That plate current may be measured by connecting a sensitivecurrent measuring device, such as a conventional galvanometer, in serieswith the plate lead, as indicated at 70. The residual gas pressure inmillimeters of mercury is typically given approximately by 10- times theplate current expressed in amperes times 10- However, since the primaryinterest is in performance of the tube, it is. usually sufficient tocalibrate the current measuring device in terms of observed performanceof the tube, rather than absolute gas pressure.

The invention is fully applicable to a wide variety of vacuum tubes, solong as the tube includes at least one grid in addition to the plate,cathode and control grid. If additional grid or grids are present, theymay be allowed to float, or tied to any suitable voltage that does notinterfere with the described tube operation. When there is a suppressorgrid of suitable form as well as a screen grid and control grid any twoof those three grids may be connected in the manner that has beendescribed for the screen grid and control grid. In the circuit for ionpumping, for example, it is suflicient that the positive grid beintermediate the negative grid and plate. Many further modifications maybe made in the particulars of the described procedures without departingfrom the proper scope of the invention, which is defined in the appendedclaims.

I claim:

1. The method of treating a vacuum tube to improve the vacuum therein,said vacuum tube comprising an evacuated sealed envelope containingcathode, plate, control grid, and at least one auxiliary grid betweenthe control grid and the plate; said method comprising connecting theplate and one of said grids that is not adjacent the plate to thenegative terminal of a direct current voltage source of the order of1000 volts,

connecting a second one of said grids that is between said one grid andthe plate to the positive terminal of such voltage source,

producing a magnetic field of the order of 2500 gauss with magneticlines of force passing through the tube transversely of the grids andplate,

and maintaining said field and said connections for a time period of theorder of to 50 hours. 2. The method of treating a vacuum tube to improvethe vacuum therein, said vacuum tube comprising an evacuated sealedenvelope containing cathode, plate, control grid, and at least oneauxiliary grid between control grid and the plate; said methodcomprising connecting the plate and one of said grids that is notadjacent the plate to the negative terminal of a direct current voltagesource of the order of 1000 volts,

connecting a second one of said grids that is between said one grid andthe plate to the positive terminal of such voltage source,

producing a magnetic field of the order of 2500 gauss with magneticlines of force passing through the tube transversely of the grids andplate,

heating the tube to a temperature of the order of 100 to 300 degreescentigrade,

and maintaining said temperature, said magnetic field 6 and saidconnections for a time period of the order of 10 to hours.

3. The method of measuring the pressure of residual gas in a vacuum tubethat comprises an evacuated, sealed envelope containing heatablecathode, plate, control grid, and at least one auxiliary grid betweenthe control grid and the plate, said method comprising supplying to thecathode a heating current of approxi mately normal magnitude, connectingthe cathode and the auxiliary grid in a circuit that includes a sourceof direct current voltage to maintain the auxiliary grid of the order of100 volts positive with respect to the cathode,

connecting the control grid to a source of negative bias of the order ofvolts with respect to the cathode,

connecting the plate in a circuit that includes a source of directcurrent voltage to maintain the plate of the order of 10 volts negativewith respect to the cathode,

and measuring the current that flows from the plate in the last saidcircuit by virtue of positively charged gas ions reaching the plate.

No references cited.

FRANK E. BAILEY, Primary Examiner.

1. THE METHOD OF TREATING A VACUUM TUBE TO IMPROVE THE VACUUM THEREIN,SAID VACUUM TUBE COMPRISING AN EVACUATED SEALED ENVELOPE CONTAININGCATHODE, PLATE, CONTROL GRID, AND AT LEAST ONE AUXILIARY GRID BETWEENTHE CONTROL GRID AND THE PLATE; SAID METHOD COMPRISING CONNECTING THEPLATE AND ONE OF SAID GRIDS THAT IS NOT ADJACENT THE PLATE TO THENEGATIVE TERMINAL OF A DIRECT CURRENT VOLTAGE SOURCE OF THE ORDER OF1000 VOLTS, CONNECTING A SECOND ONE OF SAID GRIDS THAT IS BETWEEN SAIDONE GRID AND THE PLATE TO THE POSITIVE TERMINAL OF SUCH VOLTAGE SOURCE,PRODUCING A MAGNETIC FIELD OF THE ORDER OF 2500 GAUSS WITH MAGNETICLINES OF FORCE PASSING THROUGH THE TUBE TRANSVERSELY OF THE GRIDS ANDPLATE, AND MAINTAINING SAID FIELD AND SAID CONNECTIONS FOR A TIME PERIODOF THE ORDER OF 10 TO 50 HOURS.